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U.S. Astronaut Coming Home After Epic Stay in Space

U.S. astronaut Scott Kelly and his Russian crew mate ended an unprecedented 340-day stay aboard the International Space Station Tuesday and are headed toward a parachute landing in Kazakhstan.

UPDATE: At 11:27 p.m. EST on Tuesday, NASA astronaut Scott Kelly and cosmonauts Mikhail Kornienko and Sergey Volkov safely landed in Kazakhstan as scheduled. Kelly and Kornienko had spent a total of 340 days in space as part of the International Space Station's one-year experiment.

ORIGINAL: U.S. astronaut Scott Kelly and his Russian crew mate ended an unprecedented 340-day stay aboard the International Space Station Tuesday and are headed toward a parachute landing in Kazakhstan.

NEWS: Scott Kelly, NASA's 1-Year Space Twin, Returns Today

"It's a little bittersweet," Kelly, 52, said as he turned over command of the orbital outpost to NASA astronaut Timothy Kopra, one of three men remaining on the station.

Kelly and station flight engineer Mikhail Kornienko blasted off for the first year-long stay in the station's 15-year history on March 27, 2015. They worked with eight different crewmates during their 340-day mission, a record for a U.S. spaceflight.

Accompanying Kelly and Kornienko for the 3.5 hour ride back to Earth is Russian cosmonaut Sergey Volkov, who has been aboard the station since September. Their Russian Soyuz capsule is due to make a parachute landing in Kazakhstan at 11:25 p.m. EST.

PHOTOS: Calluses to Klingons, ISS Astronaut Tells All

NASA and its partners decided to fly two crewmembers for a year on the station as a pilot program to prepare for eventual human missions to Mars lasting at least two years. Typically, crews serve aboard the station for about six months.

Kelly's spaceflight eclipses a 215-day U.S. record set in 2007 by another space station crewmember, Michael Lopez-Alegria.

The Russians, however, remain the long-duration spaceflight champs, with four Soviet-era cosmonauts spending more time in orbit than Kelly and Kornienko. The world record for the single longest spaceflight is a nearly 438-day mission by cosmonaut Valeri Polyakov, who lived on the now-defunct Soviet Mir space station from January 1994 until March 1995.

PHOTOS: These Astronauts are NASA's Longest-Duration Space Fliers

After an initial round of medical checks in Kazakhstan, Kelly is to be flown back to the Johnson Space Center in Houston for a reunion with his family and friends. During his final inflight press conference last week, Kelly said the first thing he planned to do was jump in his swimming pool.

The Russian Soyuz spacecraft carrying NASA astronaut Scott Kelly and cosmonauts Mikhail Kornienko and Sergey Volkov undocks from the International Space Station in this screengrab from NASATV's live coverage.

Scott Kelly is a NASA astronaut working for a year in space on the International Space Station. Does he have the stuff of "The Martian,"

the highly anticipated Matt Damon movie to be released on Oct. 2

, chronicling the life of a stranded astronaut on the surface of Mars? While Kelly certainly isn't on his own in space, much of the work he is doing would be useful for a trip to Mars. Here are some of the things the astronaut is working on that Mark Watney (Damon's character in "The Martian") would appreciate.

MORE: NASA's Ultimate Space Twin Experiment

The sun goes through an 11-year cycle of activity, and we are just past the peak of one of those cycles. The solar peak is a time when the sun unleashes more flares and coronal mass ejections (charged particles). When these particles hit the Earth's magnetic field, they can produce spectacular auroras.

But they also can give astronauts a higher dose of radiation.

The space station monitors radiation levels for astronauts close to Earth; in fact, one of the reasons Kelly was selected for this mission was he did not exceed the lifetime radiation levels allowed for astronauts. Radiation is expected to jump for those travelling outside of Earth's magnetic influence. Mars doesn't have much magnetic field to speak of, and the Curiosity mission is monitoring radiation levels on the surface to get more information for future human missions.

MORE: Killer Radiation: How to Protect Martian Astronauts

Working in space is a harsh business. You're busy all the time, you're stuck in a small environment with several people, and your family and friends are far away. NASA keeps close tabs on its astronauts' psychological health through measures such as doctor calls with astronauts, and

having the astronauts keep journals

during their missions. This will especially be important for Mars, as astronauts will need to be even more self-sufficient due to the time delay in communications between planets. NASA has

an ongoing comm delays study

for astronauts doing simple tasks; these tasks and their effects on astronauts will be studied as the station work continues.

MORE: Space Radiation May Harm Astronauts' Brains

Microgravity is hard on your body. NASA has its astronauts exercise for a couple of hours a day, which seems to help counteract bone loss for missions of six months. But what about a year, or longer? That's part of what Kelly's mission is supposed to answer. Bones aren't the only things to worry about, either. Muscles shrink, eye pressure increases, your sense of balance changes. Even your immune system may be affected, something that

NASA is also looking at

in detail. So while we think of astronauts as boldly doing spacewalks and experiments on station, understand that they are also part of the experiment. Their very health is being watched for the benefit of future space missions.

MORE: Space Missions Turn Astronauts' Hearts Spherical

While Watney develops a certain affection for potatoes, Kelly recently posted a picture of himself looking pretty pleased next to a floating pile of fruit. It turns out that little comforts do go a long way for astronaut morale, and any nutritionist would tell you that a varied diet of healthy foods is good for you -- not just the freeze-dried stuff the Apollo astronauts survived on during their missions. NASA has an experiment in place to see how well

astronauts are meeting nutritional requirements for their work on station

, and also for their long-term health.

MORE: Real NASA Space Tech in 'The Martian'

Astronauts are very tied to shipments from Earth right now in terms of eating ... but that is changing in a small way.

Thanks to an experiment called Veggie

, astronauts got to taste some food grown aboard the space station this summer. Lettuce, of course, does not an entire meal make. But as the movie Contact (1997) reminds us, it's through "small moves" that we learn about science. The hope is eventually this experiment will translate into a better way of harvesting crops beyond Earth. For Mars, we're even wondering how viable the soil could be to support plants.

MORE: 'Smart' LED Farming Could Make Space Veg Viable

"#ILookLikeAnEngineer on @space_station. Also a scientist, medical officer, farmer & at times a plumber," Kelly wrote with this image in August. What's more, he has to do all those things in a small space. Since every pound hoisted to space costs money, astronauts are accustomed to working in claustrophobic quarters. But NASA, concerned about its astronauts' efficiency and happiness, also has an

experiment that is supposed to look at how best to construct a living space for astronauts

. That way, the habitats designed for Mars will be suitable for long-term living.

MORE: Why 'Space Madness' Fears Haunted NASA's Past

During a recent Twitter chat, Kelly was asked if he wanted to go to Mars. He said yes, as long as he could return. Getting to Mars and back will take hundreds of days of transportation, let alone the time on the surface. The gravity on Mars is less than 40% what we experience here on Earth. And unless spacecraft design changes substantially, the astronauts will be in microgravity on the way there and back. NASA has an experiment to see

how well (or badly) astronauts work on the surface shortly after landing

, an experiment that Kelly is participating in. This will be important not only for returning to Earth, but seeing how well a crew can get adapted to Mars after being in microgravity for the transit.

MORE: 3D-Printed Bubble House Made for Mars